1. Field of the Invention
This invention relates to gimbal mechanisms and, more particularly, to a mechanism allowing controllable reorientation of an article about a spatial point. The spatial point may be selected as the center of mass of the article so that only angular momentum changes are involved in its reorientation. The gimbal mechanism utilizes large bearing surfaces between its rotatably coupled support arms and requires greater travel of these support arms, for a given amount of article reorientation, than does a conventional gimbal. Such features provide a high degree of "gimbal stiffness" and improved article pointing accuracy.
2. Description of the Prior Art
Gimbal mechanisms are well known in the mechanical arts. In its simplest form a gimbal can be found in the bearing supported freedom of a magnetic compass needle, the gimbal allowing the needle freedom to orient itself at an arbitrary horizontal (yaw) orientation with respect to its housing. Freedom in yaw and pitch, a two degree of freedom gimbal suspension system, is found in the means used to support coin operated telescopes at scenic spots of public parks. A three degrees of freedom gimbal is found in the mechanism used by attitude sensing gyroscopes in aircraft and space platforms to support a spinning inertial mass, allowing that mass to maintain a fixed spatial orientation as the airfram or space platform supporting the gimbal changes attitude.
While some gimbal systems function in a completely passive mode to support an article or device, most of them possess means to cause displacement of the gimbal elements and include position sensors to measure and control reorientation of the supported article. Fundamental to all gimbal systems is their ability to provide freedom of orientation of an axis fixed in an article or device about the axes defined by the system.
The within invention is of the class incorporating drive means and control sensor systems. Such drives and sensors may be any of a large class and are not specified herein as part of this invention although they comprise a fundamental requirement for practical application of the same.
Conventional gimbals (see FIG. 1) are generally bulky and massive since they must extend from a support or mounting base to points near the center of gravity of the articles they support. Such gimbals used for large telescopes, antennae, solar panels and machinery designed for industrial purposes are not compatible with requirements of present day spacecraft which are possessed of limited volume and severe payload weight restrictions.
A second class of gimbal is well known that conveniently attaches to the external envelope or structure of the orientable device and is thus more compact than the conventional type. A drawback to this class is that the gimbal point, i.e. the point about which reorientation is achieved, lies within the pointer mechanism itself (see FIG. 2), and large excursions of the supported device's center of gravity are required to effect device reorientation. Such excursions require large driving torques at driving points and introduce undesirable perturbations to base structure through reaction between the gimbals and their supporting base.
Problems to be solved with respect to pointing massive payload devices such as telescopes, antennae and machinery to be used for space industrialization have been studied in detail. A survey of existing pointer concepts usable with NASA's Space Shuttle and integrated payload platforms presented a number of potential solutions.
One of these utilizes an end mounted support mechanism to raise and reorient a payload device from its transport position in the Space Shuttle cargo bay to the operational condition desired. Such a system is well within current technology and utilizes well known means of energizing pointer motors or mechanisms and sensing means to control positioning (see FIG. 2).
The second solution, subject of this application, utilizes an array of support arms rotatably coupled together whose axes of rotation all meet at a common point, the center of mass of the payload device to be reoriented. Such a system, supporting a massive payload device, would involve a disturbance impulse to the system support base (Space Shuttle or payload platform) as the device is placed into operation from its transport position, but, thereafter, the only torques experienced by that support base are those arising from changes in angular momentum of the device about its own center of gravity.
This system incorporates certain desirable features of the first solution. It provides for attachment to the end or outer envelope of the supported article, yet provides for reorientation about the article's center of gravity, eliminating translational moment of inertia changes and reducing reaction torques on base structure to those resulting from the consequent changes in angular momentum.
It also incorporates large bearing surfaces between gimbal elements so that disturbance torques are distributed over a larger area than is possible with conventional gimbals. In conventional gimbals, disturbance forces are concentrated at the attachment points or axes of gimbal elements and effects adverse to stability of pointing occur when corrective torques are applied at these attach points. Overcorrection and "dither" generally occur so that "hunting" of close driven servos takes place.
Another significant feature of this invention is the perpendicular orientation of the support arm axes of rotation with respect to their supporting bearing surfaces. With such a perpendicular orientation, inerital bending moments are supported primarily by the bearings rather than by the axis pins themselves, greatly increasing gimbal stiffness. In contract (see FIGS. 1 and 2) driving joints of conventional gimbals are reacted to at the gimbal element drive means proper.
In conventional systems, a change in pitch or yaw of the payload article requires a corresponding change in angular displacement of the concerned gimbal joint. Since it is the joint that is controlled, error in the joint angle is equal to error in pointing of the article. In the mechanism of this invention, the joints traverse significantly greater angles for a given article angular displacement than do the joints of a conventional gimbal. With the same accuracy sensor, the article supported by the within disclosed gimbal can be pointed with greater accuracy than can the same article supported by a conventional gimbal. The disclosed gimbal may be identified as an exocentric gimbal to characterize its principal difference from the conventional type.
A variety of rotary and angular support devices has been in existence for many years. Gyroscope suspension means have been described in U.S. Pat. No. 3,354,726 and references cited therein, while rotary joints have been described in such patents as U.S. Pat. Nos. 2,886,262, 2,933,891, 3,284,030 and others. In none of the above, however, has the objective of achieving a suspension system such as that of the exocentric gimbal been addressed. Providing three degrees of freedom motion, with end mounting, of an extended dimension device, controllable in orientation about a selectable point, is an achievement attainable only through use of the device presented herein.